The present invention concerns a partial prosthesis meant to replace the hip's joint of the type including a metallic femoral part ended by a spherical head and a cup including an external metallic shell meant to be fitted in a milled location in the acetabulum cavity and containing a cap made of synthetic material with an internal hemispheric surface receiving the head with an easy fit.
Some prostheses of this type are known, which have a shell with an external surface which is smooth and spherical to enable it to be oriented in the cavity. Such a prosthesis, with a double articulation, has a significant clearance. However, the fit on the acetabulum cartilage or even on the sub-chondral bone can cause pain, a tear or even a penetration, which is not acceptable. To avoid these problems, a prosthesis is proposed in which the female part includes a cap made of plastic, fitted in the acetabulum cavity of the hip. In order that the clearance of the joint involves only a relative movement between the cap and the head, the interior spherical surface of the cap has a diameter sufficient to allow the head a sufficient clearance so that it has a great range of movement. This clearance, which is necessary to avoid the dragging of the cap by the head, is far from being without problems, the point of articulation not being fastened. In addition, such a cap presents a phenomenon of flow relatively fast, often called in English "cold flow," which results in a rapid loss of the shape and of the characteristics of the origin of the prosthesis.
The goal of the invention is to provide a partial prosthesis to replace the hip joint, better able to meet the practical demands than those previously known, notably because of the fact that it solves the problems cited above and provides a prosthesis which moves accurately with low friction between moving surfaces and which is compatible with body tissues.
Therefore, with this goal in mind, the invention offers a prosthesis of the type above-described, which includes a circumferential throat designed to receive serous-like synovial fluid from the body. The throat is formed between the hemispherical internal surface of the cap and the opening in which the head is inserted.
The synovial fluid is carried and distributed throughout the head by means of capillary action, insuring good lubrication during normal movements between the head and the cap, only an accidental displacement of a great amplitude (for example, after a fall) causing the shell to move relative to the bone.
The circumferential throat can have a very small depth, as a general rule smaller than a millimeter, considering that the collection of the synovial fluid and their distribution is accomplished through the forces of capillary action between the head and cup. This throat can be formed on the diameter of the internal surface and can have a depth of from one to a few millimeters. An internal protrusion on the cap can project inwardly beyond the throat to create a light retaining effect on the head once it is inserted. This protrusion needs to project only slightly on the introduction tract of the head, as its use is to insure the joining of the femoral part and femoral part having been locked in place and the cavity having been smoothed and calibrated in the opening which receives the cup, the surgeon reduces the chance of dislocation by forcing the head into the cup. A femoral part without such support could move out of the cup. The shape of the throat also increases the range of movement of the protrusion .